1. Field of the Invention
The present invention relates generally to a technique for converting images in scale or resolution, and more specifically to a technique in which the bit density of a resulting output image is substantially the same as that of the input image.
2. Description of the Prior Art
Image information, be it color or black and white, is commonly generated in a bitmap format at a particular scale, orientation .theta. and resolution K.times.L.times.b, corresponding to a desired printer output, where K is a number of spots per unit of length in one dimension, L is a number of spots per unit length in the other dimension, and b is the depth of each pixel, in number of levels. This bitmap is present for every color separation of the output device, i.e., 4 bitmaps for a 4-color output device, 3 for a 3-color, 2 for a 2-color and 1 for a black and white output device. In a common example of a black and white output, image data comprising a bitmap to be printed is provided to a printer suitable for printing at 300 spots per inch (spi) in both dimensions, at a one bit depth giving 2 levels. Many considerations drive this single selection of resolution, including the desirability of providing only a limited number of fonts (alphanumeric bitmaps) so as to use only a limited amount of storage space. Common software packages available on personal computers or for operation of input scanners for document creation also usually provide only a single resolution output. Images are commonly arranged at a fixed orientation
Increasingly, the resolution available from printers varies over a wider range of choices. Printer resolutions are available over a range, for example, from less than 200 spi to to more than 600 spi. Resolutions vary for a number of reasons, generally related to the quality of the output image. Simply printing a 300 spi bitmap at 400 spi or 600 spi is undesirable however, since the image will be reduced substantially in size on the output page or display. It would be highly desirable to provide the capability of printing any image at any resolution, while selecting the output size and orientation.
It is a common practice to implement conversion of a bitmap of first resolution K.times.L.times.b to a bitmap of second resolution M.times.N.times.d through simple pixel level and raster level operations, such as pixel doubling. When resolution is changed by integer multiples, the resolution conversion can be achieved by pixel replication. Pixel replication, however, does not best use the capabilities of higher resolution output devices. Hence in performing resolution conversion with a higher resolution output device it is desirable to perform a smoothing operation.
Scaling is an identical operation to resolution conversion, i.e., the number of pixels representing the image is increased, except that output image is represented by a lower resolution than the number of increased pixels representing the image. In other words, a conversion is implemented for a bitmap of first resolution K.times.L.times.b to a bitmap of second resolution M.times.N.times.d through simple pixel level and raster level operations, such as pixel doubling, but if the image is then directed to a K.times.L.times.b for output, it is considered scaled.
Alternatively, more elaborate methods than pixel replication have been developed to facilitate resolution conversion, some of which are described in U.S. patent application Ser. No. 07/513,415, filed Apr. 23, 1990, entitled "Bit-Map Image Resolution Converter," and U.S. patent application Ser. No. 07/588,125, filed Sep. 25, 1990, entitled "Bit-Map Image Resolution Converter Compensating for Write-White Xerographic Laser Printing," the pertinent portions of each patent application being incorporated herein by reference. Each of the above-mentioned patent applications contemplates a method of magnifying, by a predetermined magnification factor (n), original image pixels in two dimensions. The method includes the steps of selecting an original image pixel, as well as determining the binary state of both the selected original image pixel and all immediately surrounding original image pixels. Additionally, the selected original image pixel is expanded into an n.times.n array of magnified image pixels to represent the magnification of the selected original image pixel. Finally, a binary state is assigned to each pixel in the array of magnified image pixels according to the pattern of binary signals previously determined for the selected original image pixel and all immediately surrounding original image pixels. In the preferred embodiments of these patent applications, the assignment of the binary states to the pixels in the array of magnified image pixels is made according to a set of state determination rules.
Employing sophisticated resolution conversion techniques does not insure that the resulting output image will have a desirable appearance. For instance, the output image can be excessively blocky and/or contain noticeable "jaggies." Hence, smoothing operations are sometimes used in conjunction with the conversion or scaling of the image. Through use of the state determination rules in the techniques of the above-mentioned applications, smoothing operations are achieved. For example, the method of the patent application entitled "Bit-Map Image Resolution Converter" permits such operations as smoothing of edges, smoothing of half-bitting effects and smoothing of lines. Moreover, the method of the patent application entitled "Bit-Map Image Resolution Converter Compensating for Write-White Xerographic Laser Printing" permits enhancing of single bits in printing and compensating for loss of resolution in write-white printing.
The following patents also disclose techniques for achieving a higher degree of smoothing in the output image:
U.S. Pat. No. 4,280,144, Patentee: Bacon, Issued: Jul. 21, 1981
U.S. Pat. No. 4,437,122, Patentee: Walsh et al., Issued: Mar. 13, 1984
U.S. Pat. No. 4,670,039, Patentee: Neil et al., Issued: Jul. 7, 1987
U.S. Pat. No. 4,847,641, Patentee: Tung, Issued: Jul. 11, 1989.
U.S. Pat. No. 4,280,144 discloses a coarse scan/fine print algorithm. In particular the algorithm is adapted for use in a scheme in which a pixel having two levels of information is transmitted and then reconstructed into a pixel having four levels of information.
U.S. Pat. No. 4,437,122 discloses a technique for enhancing the resolution and quality of characters of a system receiving information initially in the form of image data. Through use of the technique images can be smoothed by appropriately processing unenhanced input pixels. That is, an array of subpixels can be mapped on to a selected unenhanced pixel, and the subpixels of the array can be outputted selectively as black or white to permit smoothing of the resulting output image.
U.S. Pat. No. 4,670,039 discloses a method for smoothing the display of contiguous parallel line segments in a manner to reduce the discontinuities that occur near the ends of the line segments. Smoothing is achieved by adding auxiliary dots of a lesser diameter below the larger main dots forming a first line segment in a given row and adding the same size auxiliary dots above the main dots of an adjacent line segment when the latter are in a row below the given row. The smoothing operation is optimized for multiple cases and line orientations and more than three different dot sizes can be used in the smoothing operation.
U.S. Pat. No. 4,847,641 discloses a technique for enhancing the printing of bit mapped images by piecewise matching of the bitmap with predetermined stored templates or patterns to detect occurrence of preselected bitmap features. Whenever a match occurs, an error signal is generated to produce a corrected or compensated dot or cell to replace a matched bitmap cell. In this manner the printed image of the desired bitmap image is enhanced by substituting in the original bitmap image on a piece-by-piece or cell-by-cell basis, the cells of the preselected feature with error compensated sub-cells.
None of the above references specifically contemplates a technique for resolution conversion or scaling with a smoothing operation in which bit density and corners are preserved. Since altering bit density and/or smoothing corners may be unacceptable in certain resolution conversion or scaling operations, it would be desirable to provide a resolution conversion or scaling method in which smoothing is achieved without affecting the bit density or integrity of the corners.